(1) Field of the Invention
The present invention relates generally to storing liquids in a concrete structure that can be moved while floating, and more specifically, to a barge for storing liquefied gas at sea.
(2) Description of the Related Art
Liquefied gases, such as methane, are stored either in freestanding tanks that are cylindrical, spherical, or prismatic, being made out of sheets of special steel or of thick aluminum, or else in tanks constituted by a thin membrane that provides leakproof confinement associated with a thermal insulation system constituted by blocks of foam, with the insulation system resting continuously on a support structure.
In the second configuration, the membrane is thus dimensioned solely to provide leakproof confinement of the liquid, and the mechanical strength of the assembly is provided by the external support structure which is not subjected to cryogenic cold.
French patent No. 2 271 497 discloses in particular such confinement and insulation systems suitable for liquefied natural gas (LNG) tankers and for storage on land, as developed amongst others by Socixc3xa9txc3xa9 Gaz Transport and Technigaz (Trappes, France).
Tankers fitted to receive cryogenic storage tanks are normally made of steel by specialist ship yards. For liquid methane, given that the specific gravity of the substance is low (d≈0.47), the configuration of such tankers is most unusual, and because of the high cost of making thermally insulated tanks, LNG tankers are extremely expensive. They also require many precautions to be taken in operation since in the event of liquefied gas leaking onto structural elements of the steel hull of the tanker, the steel becomes brittle and no longer withstands the stresses from the surroundings, leading to the vessel being lost.
Floating structures that are similar but made of concrete have been envisaged because concrete behaves well when put into contact with liquefied gas at very low temperature, however such structures have been designed for sailing purposes and are much bulkier and more massive than vessels made of steel, so the resulting vessels are not economically competitive with equivalent vessels made of steel. Furthermore, their draft requires them to be built in dry docks that are deep, so as to make it possible for them to be moved into deeper water after the dry dock has been flooded.
DE 2 644 856 and FR 2 366 984 disclose a concrete vessel transporting tanks located in concrete compartments. In order to minimize the wetted surface area of the vessel, it has a flat bottom on which the bottom walls of the compartments rest. The side walls of the compartments are supported by cradle type support structures.
A concrete barge is also known that was built for the Ardjuna field (Indonesia) to store liquefied petroleum gas. Gas is stored therein at a temperature of xe2x88x9245xc2x0 C. in freestanding cylindrical tanks that are thermally insulated, of circular section, and made of steel of medium thickness. The gas comprises butane and propane only. The tanks are stored on two levels: one series of six tanks is stored on deck and a second series of six tanks is stored inside the hull. Each of the tanks inside the hull rests on two cradles that form part of the concrete structure of the hull of the barge. The function of the cradles is to provide supports that come close to an isostatic system, thus minimizing the stresses generated by differential deformation between the tank and the structure of the barge and enabling the load corresponding to the weight of the tank plus its content, i.e. about 3000 (metric) tonnes, to be transferred under good conditions to the hull of the barge, which hull is subjected to buoyancy thrust over its entire wetted surface.
In that configuration, the load as distributed along the tank is concentrated via the cradles and then transferred through the cradles to the hull of the barge, thus giving rise to large concentrated forces, and then finally the load is distributed over the entire active zone of the hull that is subjected to buoyancy thrust. The barge measures about 140 meters (m) in length, 40 m in width, and 16 m in depth, and it is capable of storing about 60,000 cubic meters (m3) of gas distributed between twelve identical lagged tanks.
In the storage zone for the lagged tanks, the concrete walls corresponding to the bottom, to the sides, and to the bulkheads are thus provided with reinforcing structures including thick beams associated with concrete shells or webs that must be over-dimensioned, at least in the transition zone, so that force distribution to the remainder of the structure of the hull can take place in satisfactory manner.
Concrete barges have not yet been proposed for storing quantities of gas in excess of 60,000 m3, nor for storing liquefied gas at temperatures lower than xe2x88x9250xc2x0 C., i.e. gas other than liquefied butane or propane, and in particular for liquid methane.
With the techniques commonly used for making barges of concrete structure, giant barge building cannot be extrapolated from the technology used for the Ardjuna barge since that would require either the number of conventional tanks to be multiplied or else tanks to be made that are small in number but gigantic in size, based on free-standing technology, but in that case there would be very great difficulties of implementation, or even technical impossibilities, because of the considerable loads to be transferred via the cradles (isostatic support).
Such giant tanks, for the cryogenic temperatures of liquefied methane (xe2x88x92165xc2x0 C.) present significant shortening of the inside wall of the tank when it is cooled down, thus creating differential displacements at the supports between the tank and the structure of the barge, since the structure remains at ambient temperature. The supports become very difficult to design since they must be capable of accommodating these movements without giving rise to significant levels of stress which could create fatigue phenomena in said supports or in the tank, thus making such a barge dangerous to operate. These shrinkage phenomena exist with small tanks storing propane cooled to xe2x88x9250xc2x0 C., but they can be overcome using supports of appropriate design. Extrapolating such principles to giant tanks working at xe2x88x92165xc2x0 C. or at even lower temperatures would lead to support systems that are extremely complex, requiring major reinforcement of the concrete hull and thus requiring very large quantities of prestressed concrete to be used.
Furthermore, in spite of the good mechanical behavior of concrete, particularly when in contact with liquefied gas, the risk of micro-cracking appearing in zones of maximum stress (support cradles) can lead to water infiltrating through the solid concrete structure, running the risk of corroding the metal reinforcement inside the concrete and of degrading the performance of the insulation system, and this has dissuaded the person skilled in the art from using such concrete barges for storing liquefied methane at sea.
Such barges are subjected to large mechanical stresses from environmental conditions acting on the hull (swell, wind, currents), and also to forces that are large and very localized as created by the anchor system, which is generally situated at the four corners of the barge.
Furthermore, unlike vessels of the LNG tanker type which are generally not authorized to sail when half-loaded, and must often be either practically empty (less than 10%) or practically full (more than 85%), a floating storage facility can be filled to a level that lies anywhere in the range 0% to 100%, and it must provide very high levels of security regardless of the level to which it is filled.
U.S. Pat. No. 4,275,679 discloses concrete barges for storing liquefied gas, having concrete tanks in the form of hemispherical caps or of three-fourths spherical caps, possibly surmounted by circular cylinders extending vertically. Tanks of that shape having curvature in two directions simultaneously are difficult to make and they also require associated metal-work structures that are larger and difficult to make since the tanks do not have common side walls enabling the tanks to bear against one another and provide mutual support when placed side by side. In addition, such spherical shapes imply that a large quantity of concrete needs to be used.
The object of the present invention is to provide a barge for storing liquefied gas at sea, in particular methane, the gas being maintained at cryogenic temperatures, and in particular temperatures below xe2x88x92100xc2x0 C.
Another object of the present invention is to provide barges capable of storing large quantities, in particular more than 100,000 m3, and preferably more than 200,000 m3 of liquefied gas.
Another object of the present invention is to provide a barge capable of carrying, in particular on a top deck, a gas treatment unit, said gas being:
either received in the form of gas coming from an oil or gas well, in which case it is treated, then liquefied in specialized units, prior to being stored in the internal tanks of the barge;
or else it is received in liquid form from a tanker, in which case it is transferred on board and stored in the internal tanks of the barge, prior to being either delivered in liquefied form using other tankers or being heated in specialized units and delivered in gaseous form in underwater pipes for use at a location remote from the storage barge; or else being used on site to produce heat, electricity, or mechanical power. The energy produced using the gas can either be used on board or else exported to a remote location via underwater pipes or electric cables.
The top deck of the barge of the invention must have surface area and strength that are sufficient to receive all of the liquefaction or regasification installations, and also the equipment for producing electricity, and the total weight thereof can reach or exceed 35,000 tonnes to 50,000 tonnes.
Another object of the present invention is to provide a barge capable of being built either in dry dock in a ship yard, or else in a dry dock dug specially for the occasion, under conditions of cost and time that are economically competitive with vessels made of steel.
Insofar as the use of steel for building floating supports for storing liquefied natural gas at temperatures below xe2x88x92100xc2x0 C. and having gas treatment equipment installed on the deck thereof increases the level of risk associated with the structure becoming brittle in the event of liquefied gas escaping due to an accident in a liquefaction unit or a gas treatment unit, a problem behind the present invention is also to provide a concrete structure which presents sufficient mechanical strength in the event of such incidents, and consequently provides better safety in operation, and which can be built under better conditions concerning technical implementation and cost, in particular by minimizing the quantity of concrete used.
For this purpose, the present invention provides a barge for storing liquefied gas at sea, the barge being constituted essentially by a floating structure of reinforced and prestressed concrete containing tanks for liquefied gas.
According to the invention, said tanks are cylindrical tanks of cross-section perpendicular to their main longitudinal axes that includes a preferably circular curved portion corresponding to the bottom thereof, said portion being preferably a bottom half-circumference resting directly on the concrete bottom of the barge, with the bottom of the barge being in the form of a plurality of adjacent part-cylindrical troughs, each part-cylindrical trough having the same preferably circular partially curved section facing the bottom of each tank.
Said tank bottoms thus follow the outline of the surface and thus match continuously the shape of said barge bottom, said tank bottoms and said barge bottom being in the form of adjacent upside-down vaults.
Under the effects of currents, swell, and wind, the movements imparted to the barge give rise to dynamic variations of stress that are large, and both positive and negative in succession. In their bottom portions, the storage tanks and the barge of the invention are of a shape in the form of upside-down vaults, and in particular in the form of adjacent circular sectors, that makes it possible to use the hydrostatic pressure of the surrounding water to ensure that the cross-section of the structure is stressed essentially in pure compression, regardless of the level to which the tanks are filled, and this leads to considerable savings in materials concerning the concrete, the reinforcing structures, and the prestress means. The storage barge of the invention thus makes it possible to minimize the forces transmitted by the gas tanks to the concrete structure of the barge hull, and to avoid the load concentrations that result from using localized structural reinforcement, which would require very large quantities of concrete to be used. Stress distribution is optimal and makes it possible, throughout the working lifetime of the installation, for the assembly comprising the concrete structure and the tank to withstand firstly the pressure generated by the fluid, secondly the dynamic effects generated by the movement of the floating support under the effect of environmental conditions, and finally the various thermal stresses that are generated regardless of the level to which the cryogenic tanks are filled. Load is transferred from the liquid gas cargo to the concrete structure of the barge hull in uniform manner and the transverse stresses that arise in the concrete web are essential compression forces, which presents a considerable advantage concerning the risk of cracking and micro-cracking that exist for any concrete structure, and this avoids the risk of water migrating through the concrete of the structure and of the damage associated therewith, in terms of said reinforcement being corroded and possibly also in the insulation system if the tank is a thin membrane tank.
In a preferred embodiment, said tank is a tank of the type having a thin membrane covered on the outside in a thermally insulating complex, said complex resting directly against the concrete bottom of the barge, the concrete wall of said barge bottom being of substantially constant thickness and without any additional reinforcing structure over the entire tank-supporting zone.
The liquefied gas is thus contained in a cylindrical tank constituted by a membrane resting on an insulating complex, said insulating complex resting directly against the outside wall or the intermediate walls of the concrete structure of the barge. The concrete structure of the barge is constituted by a concrete web of substantially constant thickness in the side walls and the bottom and in register with the entire surface of the tank in question.
The use of a thin membrane tank contributes to improving load transfers and to reducing mechanical stresses in the tank since this type of tank presents stiffness that is negligible compared with the stiffness of the concrete structure of the barge, and as a result said membrane possesses very great capacity for deformation thus enabling it to follow any deformation of the concrete hull without any significant increase in stress, and regardless of whether the deformation is longitudinal bending, transverse bending, or twisting.
Associating a high performance concrete structure system, i.e. giving rise to a concrete structure hull whose ratio of buoyancy over own weight is high, with an ultra-lightweight confinement and insulation system also providing very high performance, presents the advantage while the barge is being built of requiring a dry dock that is shallower, thereby enabling construction to continue in the dry dock up to a stage that is much more advanced than has been possible in the prior art.
A larger quantity of heavy equipment can thus be installed xe2x80x9cdryxe2x80x9d, e.g. the assemblies and subassemblies required on the top deck for enabling future operation of the installations. Greater freedom is thus afforded in planning construction, and the structure can be launched at a later date, thus reducing the number of finishing operations that need to be done afloat, where such operations are generally more expensive than if done before launch. This is particularly advantageous when the dry dock is dug specially for constructing the barge, since it is then possible to dig to a shallow depth only, thus minimizing digging costs, and in addition use thereof is generally not limited by any imperative to release the dry dock in order to build the following vessel, as is usually the case with a dry dock in a ship yard.
In an embodiment, the barge has at least two and preferably at least three tanks disposed longitudinally side by side in compartments of the concrete structure of the barge and separated by vertical side walls of concrete, of structure and thickness not less than those of the concrete wall of said concrete bottom of the barge, and without any additional reinforcing structure.
Advantageously, the side walls of the tanks come against the surfaces of the side vertical walls of the compartments of the concrete structure of the barge in which said tanks are confined, so that the walls of two adjacent tanks which bear laterally against the same vertical intermediate wall between their compartments support each other mutually.
According to another characteristic of the present invention, the anchor points for the prestress cables of said prestressed concrete are situated outside said curved bottom wall of the concrete structure, and preferably outside the side vertical walls of concrete surrounding the tanks.
Preferably, the anchor points of the prestress cables are situated, in the cross-section plane of the barge, at the top ends of the side vertical walls of concrete surrounding the tanks. In this position they are easily accessible, and the prestress cables can be put under tension at the most optimum moment during building of the concrete barge, thus allowing manufacture of the insulation system and of the tank to start long before the concrete structure has been finished. The anchor points of the prestress cables parallel to the axis of the barge are situated in adjacent non-cryogenic storage zones that are reserved for consumables, fresh water, ballast, or indeed technical premises.
With giant barges, the thickness of the walls of the compartments in the concrete structure that confine said tanks, which thickness generally does not exceed 70 cm, makes it possible nevertheless to superpose sheets of metal reinforcement and sheaths for various prestress cables in such a manner that the cables used for prestressing the vault can be raised to the top portion of the concrete structure of the barge. By operating in this way, when building the concrete structure, it is possible very early on to release the cryogenic storage zone in which the tanks are to be installed, and in particular where the confinement membranes and the thermal insulation systems are to be installed, thus making it possible very significantly to reduce the overall time required for building the barge, since the insulation system which constitutes the element that is the most difficult and the most lengthy to install, can be begun at a much earlier stage during construction. The insulation system associated with its membrane must be assembled on site using individual prefabricated panels each measuring a few m2 in general. The membrane made in this way is assembled by welding while it is directly in position, whereas in the prior art tanks have been prefabricated and installed using a minimum number of packages and thus of hoists, which requires free access and consequently implies that said tanks must be installed before the top portion of the barge can be built.
In an advantageous embodiment, said cylindrical tanks and the compartments of the concrete structure in which they are confined have a cross-section perpendicular to their main longitudinal axes, the top portions of said cross-sections comprising two sloping side cants corresponding to sloping plane walls of said compartments, said sloping walls resting on the top ends of vertical side walls of said compartments and connecting said side vertical walls to a top horizontal wall for each of said compartments.
In a preferred embodiment, the walls of said tanks are applied directly against the walls of said compartments inside which they are confined, i.e. the entire wall of each of said tanks follows the outline and matches continuously the shape of the inside surfaces of the corresponding walls of its compartment in the concrete structure of the barge.
Each of said compartments, and preferably each of said tanks thus comprises:
in its bottom portion: a cylindrical trough of section that is preferably semi-circular; and
in its top portion:
two vertical side walls;
two sloping side walls; and
a horizontal top wall.
This configuration makes it possible to reduce the sloshing of liquid gas in the tank when it is not completely full.
The anchor points of the transverse prestress cables are advantageously situated at the bottom ends of the sloping walls, still outside the tank compartments, with the longitudinal prestress cables being advantageously situated in the adjacent non-cryogenic tanks or storage zones.
Advantageously, the concrete structure has bilge keels along its sides.
Also advantageously, in its top portion, above the compartments containing said tanks, the concrete structure comprises a zone made up of rectangular caissons forming the main structure of the barge and providing the stiffness of the overall structure.
The top deck of the barge has sufficient strength to receive all of the liquefaction or regasification installations, together with the equipment for producing electricity, and the total mass thereof may exceed 50,000 tonnes.
In a preferred embodiment, the concrete walls of said compartments, preferably the intermediate side vertical walls between two tanks, include a heating system embedded in the concrete. This makes it possible to limit the extent to which the wall is cooled. Heat flows through the insulation system of the tank have the effect of cooling the concrete, and this cooling is limited only by supplying heat taken either from the bottom portion of the concrete hull of the barge which is in contact with sea water, or from the top portion which is in contact with ambient air, or else by adding additional heat directly within the mass of the concrete. It is imperative to limit the extent to which the concrete is cooled since below a certain temperature the steel reinforcement becomes fragile and the strength of the structure is reduced very significantly. Said reinforcement can be selected to withstand very low temperatures, however its cost then becomes very high, so it is preferable to limit exceptional temperature drops to values of the order of xe2x88x9210xc2x0 C. to xe2x88x9220xc2x0 C., with the usual temperature of the structure preferably lying around 0xc2x0 C. to +5xc2x0 C.
Heat can be provided by incorporating electric heater cables in the concrete, or tubes carrying a hot fluid, or indeed by injecting electricity directly into the prestress cables, in particular those cables which need to be protected on a priority basis. Such additional heat can also be provided passively by mere conduction of heat along cables embedded in the concrete and in communication with the surrounding sea water.
In a preferred version of the invention, the heating system is a self-contained heating device using a thermodynamic cycle without any external heat energy being supplied thereto, the device comprising pipework embedded in the concrete and opening out into a tank having a large heat exchange area with the sea water at the bottom and on the outside beneath the concrete structure, said pipework and said tank for heat exchange with the water containing a refrigerant which circulates and transfers heat between a hot source constituted by the surrounding sea water and a cold source constituted by the mass of concrete to be heated.
The prestressed concrete barge for cryogenic liquefied gas of the invention is preferably intended for giant storage facilities having a capacity of more than 100,000 m3 of liquefied gas at temperatures below xe2x88x92100xc2x0 C., and possibly reaching or exceeding a total volume of 300,000 m3. Such a barge measures about 250 m to 300 m in length, 60 m to 70 m in width, and 25 m to 30 m in depth.
The barge of the invention may have a top deck suitable for receiving liquefaction or regasification installations, or equipment for producing electricity.
Other characteristics and advantages of the present invention appear in the light of the following detailed description given with reference to the accompanying figures.